When hill-shading is on, some map tiles are occasionally not loaded from some zoom-level down. It seams that it happens to the same tiles.
The empty tile is loaded when hill-shading and overlay is turned off/on or when I scroll out of the missing tile and back.
See next videos and bug report.
Happens to all kinds of maps (online, personal, vector) and personal overlay.
Hint: See what happens in blend-mode: overlay.